This invention relates to an all-electronic system for the detection, recognition, and positive identification of particular repetitive or non-repetitive sound patterns, and more particularly to a system for the selective monitoring and recognition of emergency signals, such as sirens, for effecting remote control of traffic signal devices.
Briefly, the present invention has primary application to the detection of emergency vehicle signals and accomplished its stated function by means of a digitally controlled, microcontroller or microprocessor pattern recognition algorithm. This algorithm functions in response to recognition of consecutive tones of different frequencies occurring in a particular pattern, established by the algorithm, to actuate a traffic signal control relay. The algorithm can be modified or preprogrammed to respond to a variety of predetermined repetitive sound patterns, while simultaneously functioning to reject unwanted or extraneous sounds and sound combinations.
The particular application and embodiment described is designed to detect and to recognize the sound of a particular operating mode of an emergency vehicle siren, such as "yelp", "wail", or "hi-low", for the purpose of controlling the operation of traffic signals at an intersection. This control is effected to make it easier and safer for an emergency vehicle to traverse the intersection. The system may be programmed to reject all extraneous sounds and sound combinations, including other siren operating modes other than a single mode to which the system is made responsive. Alternatively, the system also is capable of recognizing various operating siren modes and assigning a priority intersection control of the traffic signals to a particular mode as determined by the siren patterns. For example, priority operation could be assigned to the "yelp" mode of operation over either a "wail" or "hi-low" mode of operation, even though in absence of a "yelp" siren, one of these other modes could control the operation of the traffic signal lights.
By way of further explanation, the audio characteristics of all three of the above-identified operating modes of typical sirens including changing audio tones or sounds which have a lower frequency of approximately 500 Hz and which vary in some pattern to a frequency as high as 1600 Hz. For a "yelp" siren, this is accomplished in a sweep upwardly from the low end of this frequency range to the upper end, with a more sudden drop back down again to the low frequency for each single cycle of the "yelp" mode. This cycle then is repeated at a rate of one to four cycles per second. The exact frequency range covered and the cycle repetition rate depends on the particular model and type of siren.
For a "wall", the frequency range again is from a low frequency as low as 500 Hz, changing substantially in accordance with a sine-wave pattern to an upper frequency and back down again on a continuous basis. The repetition rate or frequency of this sine-wave variation generally is at a lower frequency than for the "yelp" operating mode. "Hi-low" sirens usually employ two frequencies with a sudden transition between the low frequency to the high frequency and back again at a rate of repetition similar to the repetition rate used in a "wail" siren.
The general utility of any system for controlling the traffic signal light at an intersection by an approaching emergency vehicle is explained in detail in U.S. Pat. No. 3,550,078. This patent discloses a system utilizing a photovoltaic detector at the traffic signal and a special high intensity lamp mounted on each emergency vehicle. The emergency vehicle lamp is directed at the detector and causes operation of the traffic signal light system to be controlled in a preestablished manner upon a sensing by the detector of an emergency vehicle lamp. This system, however, requires a modification of the emergency vehicle to add the high intensity lamp in addition to a provision of the detecting circuit at the traffic signal. If the orientation of the vehicle is such that the lamp carried by it is not properly aligned with the photovoltaic detector, the desired control does not take place, which could result in a dangerous situation for an emergency vehicle operator relying upon the anticipated control function occuring.
A number of other systems having a capability of responding to particular sounds, such as sirens or automobile horns, also have been developed. Representative systems are described in U.S. Pat. Nos. 3,568,144 and 3,735,342, both of which are designed to be mounted in a vehicle for the purpose of altering the driver of the nearby presence of an emergency vehicle siren and, in one case, also the presence of an automobile horn and a train whistle. Neither of these patents, however, make any mention of traffic signal light control.
Specifically, U.S. Pat. No. 3,568,144 describes an apparatus which is asserted to be capable of responding to the sound of a train whistle, an automobile horn, and an emergency vehicle siren. The apparatus is designed to display each of these responses separately and accomplished this purpose by means of three different channels. Each of these channels includes a band-pass filter, with one filter being tuned to the characteristic frequency of train whistles, the second being tuned to the characteristic frequency of automobile horns, and the third being tuned to the characteristic frequency of sirens. The system of this patent, however, has some significant drawbacks. First, the use of one band-pass filter to respond to the characteristic frequency of automobile horns does not work because automobile horns do not have a single characteristic frequency. The frequency of the horns varies with the make and model of automobile. In addition, most automobiles carry two horns, one of low pitch and one of high pitch, to produce a more pleasing tone. If the pass-band of the filter is made so broad so as to include the characteristic frequencies of most horns (including the low and high frequencies), the system would have no discriminating ability and would respond to many other sounds. The same reasoning holds true for a train whistle. Although the frequency range for various train whistles is narrower than various horns, the frequency range for whistles overlaps the frequency range for horns. Obviously, a siren does not have a single characteristic frequency, but sweeps a rather wide spectrum, as explained above. This frequency spectrum overlaps the frequency range of both horns and whistles. In addition, even with narrow band filters, the circuit of U.S. Pat. No. 3,568,144 has poor discriminating ability. Most street noises have a complex spectrum of sounds which contains audio components of different frequencies, and these noises are capable of causing almost constant false triggering of a circuit of the type disclosed in U.S. Pat. No. 3,568,144.
U.S. Pat. No. 3,735,342 relates to a tone responsive circuit capable of responding to the sound of an emergency vehicle siren. The system of this patent is an improvement over the systems of the patents described above inasmuch as sounds of three different frequencies must be detected within a predetermined time period, ten seconds, by means of three band-pass filters before a response is obtained from the system. An SCR sequencing circuit is used so the sounds must occur in a predetermined sequence. There is no delay time built into the sequencer, except for the inherent turn-on time of an SCR, which typically is less than 0.5 microseconds. Since the period of one cycle of a 1000 Hz tone is 1 millisecond, from a practical standpoint in audio work, a period as short as 0.5 microseconds may be considered to be instantaneous. Thus, three simultaneous tones at the proper frequencies cause the circuit to respond, as will the same three tones occurring in any sequence whatever, so long as there is at least a 1 to 2 microsecond overlap in the tones. The system of this patent does not include any effective means of rejecting unwanted sounds and therefore easily can be triggered by any broad band noise source. At best, this circuit may be considered to be a tone detector for a three-tone signal, but it essentially is ineffective as a useful sound pattern discriminator.
The system of U.S. Pat. No. 3,992,656 overcomes some, but not all, of the disadvantages of the above prior art systems. The '656 system detects siren frequencies in a sequential or reverse sequential order to control a traffic signal light. This system, however, does not have the capability of directional discrimination, nor does it respond uniquely to different sound patterns. In addition, it requires multiple band-pass filters of different narrow band characteristics within the pass-band of interest, along with timing circuits and gates, for effecting the desired control output signal in response to a preestablished sequential occurance of resonant frequencies passed by the various different band-pass filter channels.
An attempt is made in U.S. Pat. No. 4,212,085 for providing an emergency vehicle direction indicator. This patent utilizes three different sound detectors which are directionally oriented to sense the siren in different directions. No provision, however, is made for detecting a second or additional frequency within the sound pattern of each detector.
A sound pattern discrimination system for providing improved rejection of extraneous noise and also for providing directional control of a traffic signal light is disclosed in U.S. Pat. No. 4,625,206. The system of this patent is particularly suitable for detecting "yelp" sirens and employs a number of different narrow band-pass filter channels which are enabled sequentially in accordance with the anticipated pattern of the emergency vehicle siren signal. The entire system is duplicated for each of at least two different directional microphones to permit selective directional control of the traffic signals operated by the system. The various filters and sequentially enabled timing and gate circuits, however, result in a relatively expensive circuit installation which additionally must be initially tailored to respond to one of the various types of siren signals which might exist. Once the system has been configured to respond to the pattern of a particular siren, it does not respond to other different emergency vehicle siren signal patterns.
Accordingly, it is desirable to provide a sound pattern discrimination system particularly useful for controlling traffic signal lights in response to emergency vehicle sirens, which effectively eliminates false operation in response to extraneous noise, is less complex than the prior art devices, and which overcomes the disadvantages of the devices of the prior art.